A polymer may be deposited using chemical vapor deposition (CVD) techniques, including plasma enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), hot-wire chemical vapor deposition (HWCVD), and initiated chemical vapor deposition (iCVD) techniques.
iCVD is considered to be a subset of HWCVD in which selective thermal decomposition of species is achieved using resistively-heated filament wires. Substrates are generally backside cooled to promote absorption of growth species. iCVD differs from HWCVD in that an initiator in addition to the monomer is introduced into the vacuum chemical vapor deposition chamber. Initiators are generally selected such that low filament temperatures are required to generate radicals for initiation. The radicals serve as starters for polymer chains to which multiple monomer units are added.
The use of an initiator not only allows control of chemistry, but also accelerates film growth and provides molecular-weight and deposition rate control. The energy input is low due to the low filament temperature and the need only to decompose the initiator not the monomer. Radicals in the iCVD process are annihilated through termination. Both disproportionation and coupling reactions eliminate radicals and halt addition of monomer units to the chains. The recombination of radicals avoids the presence of dangling bond defects in the resulting polymeric film.
The iCVD growth of the polymer involves forming a reactive process gas by flowing a gaseous monomer into a process chamber, flowing a gaseous initiator through a heated filament into the process chamber, exposing a substrate to the reactive process gas, and thermally decomposing the process gas by flowing the process gas over resistively-heated filament wires for a period of time to deposit the polymer on the substrate.
Characteristics of the polymer structure, such as thickness and conformality with underlying structures, may be controlled by the iCVD growth parameters. These growth parameters include but are not limited to monomer source gas, initiator source gas, chamber temperature, filament temperature, growth pressure, and growth time. Generally, the growth parameters are selected such that the polymer forms a conformal layer over the underlying structure.
Initiated chemical vapor deposition is capable of producing a range of polymeric and multifunctional nanocoatings. Coatings can be made extremely thin (down to about 10 nm) on objects with dimensions in the nanometer range (e.g., carbon nanotubes). Importantly, the object to be coated can remain at room temperature, which means that nanothin coatings can be prepared on materials ranging from plastics to metals. The process is also conformal, which means it provides uniform coverage on objects which have small, complex, three-dimensional geometries.
Initiated CVD generally takes place in a reactor. Precursor molecules, consisting of initiator and monomer species, are fed into the reactor. This can take place at a range of pressures from atmospheric pressure to low vacuum. An extremely thin, conformal layer of monomer molecules continually adsorbs to the substrate surface. The initiator is broken down through the addition of thermal energy or radiative energy (UV) to form free radicals, which subsequently add to a monomer molecule and cause polymerization to proceed in a manner analogous to well-known solution polymerization. In this manner, complex substrates can be conformably coated. During the deposition the substrate is kept at a relatively low temperature, generally room temperature up to about 60° C. The process is solvent-free. The iCVD process can also use plasma excitation to generate initiating free radicals. This can be done by flowing gas-phase monomer or by atomization of the liquid monomer species through a plasma field. This can take place at a range of pressures from atmospheric pressure to low vacuum.